The first step on the way to understanding the complicated dynamics of spray is to study the behavior of isolated droplets. In many industrial and natural processes such as turbulent combustion, agricultural sprays, spray cooler, falling raindrops and cloud evolution the droplet is subjected to a chaotic unsteady external flow field. The interaction between the liquid and gaseous phases results in very intricate droplet dynamics like capillary instabilities, atomization, droplet collision and coalescence and vaporization, to name a few. In this dissertation, the focus is on shape oscillations, atomization and vaporization dynamics of pendant and sessile droplets. A droplet residing on a substrate which vibrates vertically at ultrasonic frequency will exhibit different modes of shape oscillation. The competition between capillary forces and inertia forces is basically responsible for these oscillations. However, when an acoustic force field is introduced asymmetrically around the droplet, we discover with the help of ultra high-speed imaging, a new droplet spreading phase. This new method of droplet manipulation could nd application in micro fluidics and lab-on-a-chip systems. By lading the droplet with nanoparticles, the spreading rate can be easily controlled. The spreading phase is followed by an atomization phase where surface ligaments grow to disintegrate into daughter droplets; the intensity of atomization is found to decrease with increase in fluid viscosity. The ability to control atomization characteristics of droplets by lading them with nanoparticles is a powerful technique that may be implemented in spray coolers and combustors to control the spray characteristics or combustion efficiency. Both the spreading and ligament dynamics have been theoretically simulated and the physics behind the observed trends is explained. The growth rate of the ligaments is found to be governed by Weber number modified to include the acoustic pressure level of the standing wave. The frequency of ligament breakup is found to decrease with fluid viscosity and this observation is adequately supported by a theory developed based on the evolution crater on the droplet surface. Turning now to the pendant droplets, by decomposing the droplet shape into Legendre modes, the shape oscillations exhibited by a droplet hanging from the junction of cross-wire placed at the center of an air jet is studied. Both high-speed imaging and hot-wire anemometry are employed. The driving force of oscillation of droplets subjected to the air jet is proved to be the inherent pressure fluctuations in the jet. The effect of surface tension, viscosity and Reynolds number on the shape oscillation level has been examined. The first experimental evidence of viscous attenuation of lower frequencies in a particular mode in glycerol/water mixture is reported. A theoretical model was developed to simulate the droplet shape oscillations induced by different ambient flow fields like pulsatile flow, vortical flow and flow with broadband energy spectrum. The time of interaction of the droplet with an eddy in the flow is found to be very crucial in determining the amplitude of oscillation of the droplet. The shorter the interaction time, the higher are the chances of the droplet oscillation being pushed into resonance. Finally, the heat transfer and droplet regression dynamics of pendant droplets in a hot air stream of various chemical compositions (like conventional fuels, alternative fuels and nanosuspensions) have been experimentally analyzed using high speed imaging. The droplet is deployed at the junction of cross-wire at the centre of a vertical air jet. A hybrid timescale has been proposed which incorporates the effects of latent heat of vaporization, saturation vapor pressure and thermal diffusivity. This timescale in essence encapsulates the different parameters that influence the droplet vaporization rate. The analysis further permitted the evaluation of the effect of various parameters such as surrounding temperature, Reynolds number, far-field vapor presence, impurity content and agglomeration dynamics (nanosuspensions) in the droplet.

Este trabalho trata da modelagem de sinais ultrassônicos gerados por transdutores circulares (plano e côncavo) no modo de pulso-eco, inspecionando uma região de interesse predeterminada. Para essa análise, dois modelos da resposta impulsiva de um transdutor piezelétrico foram implementados: um do transdutor circular plano e outro do transdutor circular côncavo focalizado. Este último sendo o modelo proposto pelo autor com uma geometria baseada em anéis concêntricos como elementos de área do emissor. A adição de diversos anéis concêntricos, deslocados ao longo de seu eixo e de raios sucessivamente menores, permitiu calcular o campo acústico, gerado por uma abertura côncava, bem como seu eco refletido. A resposta impulsiva de cada anel resulta da diferença entre as respostas impulsivas calculadas para um emissor circular grande e um pequeno. O modelo implementado para o cálculo dos sinais de eco foi utilizado na varredura de uma região contendo um conjunto de pontos com refletividade acústica igual a 1. A reconstrução da imagem foi feita com esses sinais aplicando-lhes a técnica de regularização de Tikhonov. A qualidade das reconstruções das imagens obtidas foi avaliada e comparada a das imagens convencionais. Dentre as métricas de avaliação das imagens reconstruídas estão a influência na variação da velocidade de propagação da onda acústica no meio, a extensão e a discretização do grid e o parâmetro de regularização alfa. Todas as reconstruções foram analisadas segundo o Erro Médio Quadrático (MSE). Finalmente, ensaios experimentais foram conduzidos para a obtenção de A-scans (imagens em modo de amplitude) os quais foram inseridas no modelo teórico para a reconstrução de imagens e analisadas pelo MSE. / This work deals with the modeling of ultrasonic signals generated by circular transducers (planar and concave pistons) in pulse-echo mode, inspecting a predetermined region of interest (ROI). For this analysis, two models of the impulse response of a transducer were implemented: one using a plane piston transducer and the other, a model proposed by the author of this work, using a concave transducer with concentric rings as elements of the emitting area. The addition of several concentric rings moved along its axis allowed us to calculate the acoustic field generated by a concave opening and the echo reflected from each point in space. The impulse response of each ring represents the difference between the impulse responses calculated for a large circular transmitter and a small one. The model implemented for calculating the echo signals is used to scan a region, within a ROI, containing a set of points with acoustic reflectivity of 1. Simulations of the regions are made with these signals by applying the Tikhonov regularization method. To evaluate the quality of image reconstruction, the images are compared with the conventional images. Among the metrics to evaluate the reconstructed images are the influence of the variation of the acoustic wave propagation in the media, the grid range and discretization and the parameter of regularization alpha. All of the image reconstructions were analyzed through the Mean Square Error (MSE) criterion. Finally, experiments were conducted in order to obtain A-scans which were then re-inserted in the theoretical model to reconstruct and analyze the images.

No / New representations and efficient calculation methods are derived for the problem of propagation from an infinite regularly spaced array of coherent line sources above a homogeneous impedance plane, and for the Green's function for sound propagation in the canyon formed by two infinitely high, parallel rigid or sound soft walls and an impedance ground surface. The infinite sum of source contributions is replaced by a finite sum and the remainder is expressed as a Laplace-type integral. A pole subtraction technique is used to remove poles in the integrand which lie near the path of integration, obtaining a smooth integrand, more suitable for numerical integration, and a specific numerical integration method is proposed. Numerical experiments show highly accurate results across the frequency spectrum for a range of ground surface types. It is expected that the methods proposed will prove useful in boundary element modeling of noise propagation in canyon streets and in ducts, and for problems of scattering by periodic surfaces.

Pour limiter la pollution sonore et répondre aux exigences des clients, le secteur automobile est contraint pour la prochaine décennie à diminuer drastiquement les émissions sonores de ses véhicules. Le Groupe Moto Propulseur (GMP) constitue l’une des principales sources de bruit et les solutions palliatives couteuses ne permettront plus à terme le passage des normes acoustiques. L’optimisation du comportement vibro-acoustique de telles structures par un travail détaillé de la conception reste le meilleur levier. Dans ce contexte, les ingénieurs sont à la recherche de méthodes expérimentales d’identification et de caractérisation de sources acoustiques particulièrement efficaces et pertinentes. En ce sens, la principale attente est la reconstruction précise des champs acoustiques sur des structures à géométrie complexe en présence d’un environnement industriel contraignant et non contrôlé. Dans la continuité de la méthode u-iPTF (inverse Patch Transfer Functions) précédemment développée, la première partie de ce travail de doctorat s’est attachée à uniformiser le principe du volume acoustique virtuel sur lequel elle repose afin de proposer la formulation d’une méthode alternative, dénommée m-iPTF, dont le principe théorique permet de répondre intégralement au cahier des charges du partenaire industriel. En effet, contrairement à la méthode u-iPTF, la méthode m-iPTF s’affranchit de la mesure des vitesses de particules d’air, encore peu robuste et coûteuse, et utilise uniquement des pressions acoustiques. Une première étude numérique a cadré le domaine de validité et de mise en application de la méthode. Elle a été suivie par trois campagnes expérimentales de complexité croissante dont une application en conditions industrielles standards, celle d’un GMP complet fonctionnant sur banc moteur. L’ensemble de ces études a permis de démontrer l’adéquation de la méthode à un milieu industriel et d’illustrer toutes les potentialités offertes par la reconstruction 3D de champs pariétaux pour l’optimisation de structures complexes (discrimination et hiérarchisation de sources, corrélation et couplage à la simulation numérique...). / To limit the sound pollution and to meet customer requirements, the automotive sector is constrained for the next decade to drastically reduce the noise emissions of its vehicles. The powertrain is one of the main sources of noise and expensive workaround will not be sufficient in the future to meet acoustic standards. The optimization of the vibro-acoustic behavior of such structures by a detailed design work remains the best way. In this context, engineers are looking for an efficient and relevant experimental noise source identification and characterization method. In this sense, the main expectation is the precise reconstruction of all the acoustic source fields on geometrically complex structures and in the presence of a restrictive and uncontrolled industrial environment. In the continuity of the u-iPTF method (inverse Patch Transfer Functions) previously developed, the first part of this PhD work was to standardize the principle of the virtual acoustic volume on which it relies and to propose the development of an alternative method, called m-iPTF, whose theoretical principle allows reaching all the specifications of the industrial partner. Indeed, contrary to the u-IPTF method, the m-iPTF method overcomes the measurement of air particles velocity, expensive and not yet robust, and uses acoustic pressure only. A first numerical study framed the scope of validity and application of the method. It was followed by three experimental campaigns of increasing complexity with a final application in industrial standard conditions, i.e. a powertrain running on a test bench. All these studies have demonstrated the suitability of the method in an industrial environment and illustrated the potential of the 3D reconstruction of parietal fields for the optimization of complex structures (discrimination and ranking of noise sources, correlation and coupling with numerical simulations...).

L’analyse vibratoire constitue une part très importante des moyens de mesures pour la surveillance et la détection des défauts mécaniques des machines tournantes. Le positionnement des accéléromètres est stratégique et contribue fortement à la réussite du diagnostic ; la proximité du capteur de l’élément défaillant est une condition très utile, mais pas toujours réalisable. La corrélation entre le bruit émis par une machine et son état est assez étroite et montre l’apport des mesures acoustiques pour l’optimisation du diagnostic. L’imagerie acoustique, très appliquée pour détecter des sources dans le domaine du transport, avec ses multiples méthodes (holographie, beamforming, etc…) peut être un moyen pour remonter aux défauts mécaniques. Dans cet objectif, plusieurs stratégies basées sur l’algorithme de beamforming sont développées. Premièrement, des indicateurs communément utilisés pour le diagnostic des machines sont visualisés en fonction de l’espace. Le kurtosis permet de localiser les sources impulsives qui peuvent être reliées à un défaut. De nouveaux indicateurs basés sur le spectre d’enveloppe des signaux focalisés sont également mis en place pour détecter les défauts de roulement de bague interne et externe. D’autre part, la moyenne synchrone angulaire est utilisée pour extraire le champ acoustique synchrone avec la rotation d'un composant de la machine. Les sources reliées à un défaut sont affectées au champ résiduel et peuvent être identifiées dans les cartographies. Enfin, une nouvelle méthode d'imagerie acoustique qui exploite les fonctions de transfert vibroacoustiques entre des accéléromètres positionnés sur la machine et une antenne acoustique est développée. Elle permet d'obtenir des cartographies de la pression rayonnée sur une surface de la machine uniquement à partir d'accéléromètres. Son applicabilité à la détection de défaut est également démontrée sur un banc à engrenages. / Vibration analysis is mainly used in condition monitoring and fault detection of rotating machine domain. The success of the diagnosis is strongly related to the position of the accelerometers. However, the machine geometry sometimes prevents the sensors to be placed close enough to the faulted part causing the diagnostic failure. The sound emitted by a mechanism and its condition are related. Using microphones to optimize condition monitoring is then justified. Acoustic imaging techniques (acoustic holography, beamforming, etc…) are mainly used as a source localization and quantification tool but they can be turned into a powerful diagnosis tool. Several strategies based on the beamforming algorithm are developed in this work. Firstly, diagnosis features commonly used in condition monitoring of rotating machinery are mapped as a function of space. Kurtosis allows localizing impulsive sources which eventually can be related to a mechanism failure. New features based on the squared envelope spectrum of the focused signals are also introduced. They aim toward the detection of inner and outer race fault in roller element bearings. On the other hand, angular synchronous average is used to extract the acoustic field synchronous with one component rotation. The sources related to a fault are localized in the residual field mappings. Finally, a new imaging technique based on the vibroacoustic transfer functions between a few accelerometers placed on the machine and the microphone array is developed. It allows obtaining the mappings of the radiated pressure on the machine surface only thanks to the accelerometers. It is tested as a fault detection tool on a test bench

Un jet heurtant une plaque fendue peut générer, dans certaines configurations, des nuisances sonores. En effet, l’interaction de l’écoulement et de l’obstacle au niveau de la fente, sous certaines conditions, donne naissance à une perturbation remontant l’écoulement et pouvant contrôler son détachement à sa naissance. La perturbation produite par cette boucle de rétroaction optimise le transfert d’énergie du champ aérodynamique du jet vers le champ acoustique rayonné. Afin d’appréhender la dynamique tourbillonnaire, d’analyser les couplages entre cette dernière et les émissions sonores générées et de mieux comprendre les phénomènes responsables de ces nuisances, un dispositif expérimental basé sur de la métrologie laser a été réalisé. Ce système permet, d’une part, la génération de l’écoulement et la maitrise de ses paramètres (confinement, vitesse, forme,…) et d’autre part, la réalisation de plans lasers et de mesures par imagerie de particules (PIV). Ainsi le travail présenté dans ce manuscrit concerne les couplages qui existent entre la dynamique de l’écoulement heurtant une plaque fendue et les champs acoustiques générés. Les mesures de champs cinématiques d’un jet plan heurtant une plaque fendue par Vélocimétrie par Images de Particules (PIV) sont réalisées simultanément avec des mesures de champs acoustiques. Après avoir caractérisé les écoulements étudiés, on présente par des graphes spatio-temporels, les corrélations entre les signaux acoustiques et les vitesses de l’écoulement depuis la sortie du jet jusqu’à son arrivée à la plaque fendue. Ces corrélations sont calculées de deux manières : à partir de signaux bruts dans un premier temps, puis, dans un second temps, avec une méthode de pré-blanchiment (terme anglo-saxon : ‘’pre-whitening’’). Cette méthode vise à mettre en exergue l’existence d’une instabilité globale du jet qui existe dans les signaux analysés. Cette instabilité est importante pour la boucle de rétroaction des sons auto-entretenus, mais quasiment masquée devant les phénomènes principaux dominants (tourbillons primaires) dans le calcul des inter-corrélations. / Self-sustaining sounds related to aero-acoustic coupling occurs in impinging jets when a feedback loop is present between the jet exit and a slotted plate: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate are phase locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and result in periodic coherent structures. The period is determined by the convection speed of the coherent structures and the distance between the nozzle and the plate. Simultaneous measurements of the velocity fields and the acoustic waves in a plane jet impinging a slotted plate were performed using time-resolved particle image velocimetry (PIV) and a microphone. A better understanding of the flow physics and the aero-acoustic coupling are obtained thanks to spatio-temporal cross-correlations between the transverse velocity and the acoustic signals. Cross-correlations are calculated using two different methods: classical analysis of the original signals and by developing a pre-whitening technique. The latter method is useful for analyzing small random signals superimposed on a high amplitude pure tone.

Near-field acoustical holography is a standard method for sound source visualization and localization. In this thesis, commonly used and newly published near-field acoustic holography methods are introduced and analysed. In addition, regularization methods are presented together with finding options of optimal regularization parameter. Based on theory, a LabVIEW library is built containing four implemented near-field acoustical holography algorithms and two regularization methods. To verify the correctness of implementations, a testing application has been made. This application allows testing of implemented algorithms with simulated or experimentally measured data. The correctness of implementation is verified, and algorithms are compared to each other with respect to accuracy and speed of calculation.

This paper presents parameters of evaluation of acoustic quality of the space. It is divided into parts presenting physical principle of the origin and movement of the acoustic signal, principles of its processing with current technology and properties of the acoustic field. This is followed by an analysis of the musical part and notes on psychoacoustics. The document contains a description of relevant parameters of acoustic spaces and way in which we can reach desired results, including material analysis. The paper mainly focuses on description of relevant parameters of three acoustic spaces which were measured. The last part of the work is a program for elementary acoustical measurement, which can be provided by means of commonly accessible equipment such as a notebook or a personal computer.

A wind tunnel from the 1950s located in Bromma, Stockholm, once used for military research is today used for commercial activities. Today the tunnel is used for indoor wingsuit flight and the facility has an interest in reducing the overall noise generated by the tunnel. Acoustic measurements indicate noise problems in the 50 Hz range. A 3D and a 2D model was structured with physical measured dimensions of the wind tunnel for simulations.Results indicate that a 37 dB decrease of the second higher order azimuthal spinning mode was achievable with the same number of monopole anti-sources as fan blades with each individual monopole modeling an enclosed loudspeakers. This acoustic mode was identified as the most problematic due to the cut-on frequency for the geometry coinciding with the fundamental blade pass frequency of the fan source during normal operating conditions. / På 1950-talet byggdes en vindtunnel i Bromma, Stockholm kallad "LT1". Vindtunneln användes då för militär utveckling för bland annat flygförsvaret. Idag drivs den efter en period utan användning kommersiellt för flygning av ekorrdräkt så kallad "Wingsuit" inomhus. Området kring tunneln har kommersialierats under tiden den var ur drift och omgivande verksamheter påverkas av ljud från vindtunnelns verksamhet varför tunnelns verksamhet söker förbättringsmöjligheter avseende bullerdämpning. Akustiska mätningar i och utanför tunneln indikerar ett problemområde kring frekvensen 50 Hz som härstammar från den stora fläkt som skapar flödet i tunneln. Vindtunneln modellerades i 3D och 2D med hjälp av fysiskt uppmätta dimensioner på plats. Modellerna användes sedan för att kunna numeriskt simulera och beräkna möjligheten att dämpa ljudet från fläkten med hjälp av aktiv kontroll. Resultaten indikerar att en 37 dB ljudreduktion är möjlig av andra ordningens högre akustiska snurrande mod. Detta var möjligt att uppnå med lika många anti-källor modellerade som monopoler vilket motsvarar slutna högtalare, som fläktblad. Denna akustiska mod identifierades som den mest problematiska eftersom cut-on frekvensen för tunnelns geometri sammanfaller med den fundamentala bladpass-frekvensen för fläkten under normal drift.